Musumi Nishikawa aka Masuyo Mamiya (Hisako Kyouda) is the hidden villainess of "The Old Blue Castle Investigation Case", episodes 6.02 and 6.03 of Case Closed (airdates February 22 and March 1, 1999).
Musumi Nishikawa was a maid for the esteemed Mamiya family and was believed to have died in a fire that broke out one of the towers of the Mamiya mansion, killing Masuyo Mamiya's eldest daughter and several of her friends as they were celebrating Ms. Mamiya's return from overseas.
But in actuality, not only had Musumi survived the fire, but had actually started it from within the tunnel system underneath the mansion. As revealed in the episode's climax, Musumi had killed Masuyo and received surgery in order to impersonate her, and killed Masuyo's daughter and several others in the tower fire to prevent them from exposing her. Musumi's reason for the facade was to search the tower for a secret treasure hidden inside the Mamiya castle. To prevent others from searching for the treasure, the evil Musumi killed the assistant of Mitsuru Mamiya (the husband of the Mamiya's eldest daughter) to make it appear that the Mamiya estate was cursed.
Conan Edogawa, Professor Hiroshi Agasa, and his friends arrived at the Mamiya castle after becoming lost while driving home from a botched camping trip and were invited to stay the night by Mitsuru in hopes that Agasa could find out where the treasure was. Throughout the stay, Musumi put on the act of a senile elderly woman, acting as though she believed "her" daughter to still be alive and angrily accusing Mitsuru of having only married his wife for her money. When Conan entered the secret passageway that led to where the real Masuyo Mamiya's body was, Musumi attacked and abducted him, doing the same to Agasa and Genta Kojima when Conan's friends went searching for him.
Eventually, Ai Habara and Ayumi Yoshida came across the skeletal corpse of Masuyo, and it was then that Ai realized Musumi's status as an impostor. When Musumi came up to the girls, Ai and Ayumi fled from her, but were caught up to by the villainess as they climbed out of the tunnels. After confessing to having killed Masuyo and gotten surgery to impersonate her, the evil Musumi prepared to kill Ai and Ayumi, but was stopped by Conan, having been freed (along with Genta and Agasa) by Mitsuhiko Tsuburaya. Agasa and Conan then revealed that they had already figured out Musumi's evil scheme, as she had earlier revealed a knowledge that passport sizes had changed in size six years prior, despite the real Masuyo having not left the castle in ten years.
As Musumi prepared to flee, however, Conan revealed that he had solved the treasure mystery and deduced that the passageway to the treasure was located behind a portrait of the Mamiya grandfather. Musumi eagerly raced to the passage, only to find that the "treasure" was the tower the passage led to and the beautiful scenery it provided a view of. At that, Musumi broke down in dismay at having committed mass murder and destroyed her face for nothing before being arrested offscreen.